INFLUENCE OF POLYDISPERSITY ON PROTEIN CRYSTALLIZATION - A QUASI-ELASTIC LIGHT-SCATTERING STUDY APPLIED TO ALPHA-AMYLASE

The early stages of the crystallization process of porcine pancreatic alpha-amylase were investigated by quasi-elastic light scattering. It is shown that at 288 and 293 K the diffusion coefficient does not mono tonically change with increasing protein concentration but passes thro ugh a maximum at 10 mg ml-1, In supersaturated solutions, prior to nuc leation, the protein is strictly monodisperse. Nucleation induces the formation of aggregates and a polydispersity of, for example, 18% for an initial supersaturation C/C(e) = 5.8. Monodispersity is restored af ter the nuclei have grown and partially consumed the solute. On the ot her hand, polydispersity increases up to 20% at 298 K if the protein c oncentration decreases to 3-4 mg ml-1, values at which the solutions a re under saturated. When the protein concentration exceeds 5-6 mg m-1 the protein becomes monodisperse again. These results, confirmed by th ose of another system we are studying (bovine pancreatic trypsin inhib itor), are at variance with the statements that supersaturation is alw ays at the origin of aggregation and polydispersity, and that in under saturated solutions the diffusion coefficient should remain constant f or obtaining crystals once the solutions are supersaturated.